Low intensity optical signals, especially sub-photon signals or signals that maintain some unique quantum properties such as entangled photons, are sensitive to loss and added noise. Switches are commonly used as a tool to control optical signals, for instance in optical communication networks. The switch technologies tend to either be slow (e.g. 1 MHz switching) but capable of having low losses (e.g. <1.5 dB), or fast (>100 MHz) but more lossy (e.g. 3 dB). Slow switches are adequate for many purposes such as network reconfiguration, but some applications may require high speed switching or modulation. Optical amplifiers can be used to compensate for loss, but small optical signals, especially quantum signals or low-photon optical pulses that do not have a precisely known temporal location prior to measurement (such as a lidar return signal), can get swamped by the added noise of such amplifiers eliminating such amplifiers from consideration. Such signals would benefit from a low loss, low noise, and high speed switch.
One switching method that is capable of low loss, say <1 dB of insertion loss, that can also be fast and low noise is the use of cross phase modulation (XPM) in optical fiber. Such switches have been demonstrated to work for quantum signals in the 1310 nm band using pump photons in the 1550 nm band [M. Rambo, et al. “Low-loss all-optical quantum switching.” Photonics Society Summer Topical Meeting Series, 2013 IEEE. IEEE, 2013]. This choice is convenient since 1550 nm is a well-developed technology because of its use in telecommunications, where 1550 nm is of special significance because it is the lowest loss wavelength in typical optical fibers. It is helpful for the pump wavelength to be longer than the signal wavelength since this reduces spontaneous Raman photon generation which is an important noise source. Furthermore, 1310 nm is still a reasonably low loss wavelength in fiber. 1550 nm and 1310 nm are separated by about 36 THz in frequency space. This large frequency separation in combination with the pump being at the longer wavelength which creates reduced Raman scattering allows the Raman scattering levels to be well controlled.
The XPM switch has been used thus far in applications where the input pulse location is known. The pump location can thus be appropriately set with respect to the incoming input pulse location. In many applications such as pulse characterization or lidar return signal evaluation the pulse location and/or its temporal profile are not known.
The XPM switch would be much more useful if it worked for signals in the technologically important wavebands from 1500-1610 nm. A straight forward design change of the previously demonstrated XPM switches would then use a ˜2 μm pump laser to maintain a similar optical frequency separation between the pump and signal in order to maintain low Raman noise. This is inconvenient since 2 μm is a less well developed technology, and it is fairly lossy in Silica optical fibers thereby limiting the length of the longest nonlinear fiber that can be used. Moreover it is desirable if the pump and signal wavelengths have a similar group velocity leading to a low group velocity mismatch (GVM), that is to say that pulses at the pump and signal wavelength propagate at nearly the same velocity, and standard fibers like SMF-28e have large GVM between these two wavelengths since they are both on the same side of the zero dispersion wavelength and separated by a large wavelength difference. For a short pump pulse, a given GVM in ps/m, and a given desired switching window τsw is ps, the length of nonlinear fiber LNL is limited to LNL≤τsw/GVM. For a given switching window a large GVM thus limits the length of nonlinear fiber. Put another way, the walk-off delay induce by GVM is τD=LNL·GVM and the switching window can be estimated as τsw2=τD2+τpump2, where τpump is the temporal duration of the pump pulse. The minimum switching window occurs when using a pump pulse much shorter than the group velocity walk-off delay in which case the switching window is the group velocity walk-off delay.
To obtain a desired π phase shift a pump power of at least Ppump=π/(γ·LNL) is needed, where γ is the nonlinear parameter of the fiber in units of (W·km)−1. Thus a large GVM leads to high required pump switching powers. High powers can be inconvenient, and eventually there are limitations due to for instance optical fiber fusing that prevent too much average power from being injected into the fiber.
To address the large GVM one could try to engineer a specialty fiber as the nonlinear fiber, for instance engineering the fiber to shift the zero dispersion wavelength to, say, 1750 nm which is in-between the pump and signal wavelength. Or one can engineering the fiber to have higher levels of nonlinearity. Such fibers which would likely have large splice loss to the standard fiber used for the rest of the switch components and a large splice loss to the input signal that is likely transmitted in a standard fiber. Alternatively, standard fiber could be used in the switch and a dispersive element with opposite dispersion as the standard fiber could periodically be inserted into the nonlinear fiber to ‘readjust’ the pulse timing between the pump and signal thus making the average GVM small (periodic dispersion compensation). This solution would also adversely impact the insertion loss of the signal, which is a critical metric especially for sub-photon quantum signals such as entangled photons.
What is needed is a low loss, high speed, low noise switch that can work near the communications wavelength band (e.g. 1550 nm) but can be built using more established technology. Ideally the nonlinear fiber should be a standard fiber for this wavelength such as the Corning SMF-28e fiber. The switch should be compatible with efficiently handling cases where the input signal arrival time is unknown. The switch can optionally be configured to have a periodic transfer function in the optical frequency of the input signal. Ideally the switch can control the splitting ratio of an input optical signal to one or more output ports, where a single output port is essentially a variable attenuator or an on/off switch. For cases with two output ports, the pump power can control the splitting ratio between the two ports, including full switching where 100% of the light exits one output port or the other, or partial switching where the input light is deterministically split between the two ports.